Boring and honing Harley-Davidson cylinders Part 1

Harley-Davidson cylinders suitable for reboring

All Harley-Davidson and Buell air cooled cylinders can be bored and honed, and there are a number of reasons that we might need to do this. They might just be worn out through high mileage, or (unfortunately far too common), prematurely worn out due to overfueling as the result of bad tuning, or inadequate oil change intervals. They may be scored as a result of a component failure in the bottom end (big end bearing, lifter axle, and inner cam bearing failures are the main culprits). The Evolution cylinders can distort in use if abused, and this is typically the rear cylinder on the Big Twins and Sportsters, and the front cylinder on the Buells. The Buell front cylinder is more stressed because of the way that the engine hangs from the top front motor mount, and this is exasperated by heavy landings after wheelies. Or, and this now forms the bulk of our cylinder work, we may wish to achieve a useful performance gain by increasing the bore size and fitting bigger bore and/or higher compression pistons. The Twin Cam 88 to 95, the Twin Cam 96 to 103, and, of course, the Sportster 883 to 1200 conversions are popular examples. Many of our Harley-Davidson Twin Cam and Sportster performance upgrades utilise cylinder boring and honing as part of the process.

There are a couple of other things to note, regarding how far we can go with this. Harley say that new cylinders are required for the Buell XB9, XB12 and the Sportster XR1200 when things wear out. This is incorrect, as these cylinders are constructed the same way as the Evolution Sportster 1200 cylinders. (They are identical, apart from the exterior finish). Harley just do not have oversize pistons available for them. However, Wiseco do, and we can safely use the same oversizes in these cylinders as we can in the regular XL 1200.

The Harley-Davidson Twin Cam 88 and Twin Cam 96 cylinders are the same, and have the same bore size of 3.750". Boring these out to 3.875 gives us 95" and 103" respectively, and these are then the same as the cylinders that H-D sells as "off the shelf" for these capacity upgrades. Boring and honing the original cylinders, in the way that we describe later, is both cheaper and more accurate. It is also the case that these stock Twin Cam cylinders can be safely bored to 3.938" diameter. This gives 98" and 107," respectively, and there have been no reported reliability issues.

Harley-Davidson Panhead and the earlier, thin base flange, Shovelhead cylinders, can only be safely taken out to +.070 oversize. The later, thick flange, 1200 cylinders, which are of the same construction as the 1340 ones, can go out to +.090 oversize. After market cylinders from S&S, Axtell, and Sputhe can also be "rebored", subject to piston availability.

The principles of Harley-Davidson cylinder boring and honing

The fundamental principles of these machining operations are no different to those required of most other motorcycle, automotive, or aeroplane engines. There are, however, some peculiarities that must be accommodated, and these sound engineering principles, if adhered to, will give a reliable result. None of this is rocket science, and it has all been understood for over a century, but we have seen some appalling workmanship, even from factory produced components.

So, here it is. The cylinder must be straight, round, have the correct surface finish, and the axis of the bore must be at perfect right angles to the cylinder base. We could be forgiven for thinking that something this simple would be easy to achieve, but it requires specialist machinery, very accurate measuring equipment and strict adherence to very tight tolerances. The two machining operations involved are boring and honing.

Boring will remove material relatively quickly, generate a straight and round hole, and can also ensure that the bore axis is square to the cylinder base. But it cannot produce the correct surface finish or consistently hold the tight tolerance on size that we need to ensure. Honing is a very accurate, but slow, process (similar to grinding). Honing can consistently hold a tight tolerance, also produce a straight and round hole, and impart the desired surface finish conducive to good piston ring seal and long life. However, because the honing head "floats", it follows the existing bore axis and if this is not right, honing will not correct it. Also, because it is a relatively slow finishing operation, it would take far too long to remove substantial amounts of material. Before we look at these two operations we first need to discuss the use of torque plates and the measuring equipment. If we can't take accurate measurements, the machining processes will not achieve what we need.

Harley-Davidson torque plates

When the Harley-Davidson Evolution Big Twin was introduced for the 1984 model year, this represented a radical departure from the earlier Harley engines. Most notably the use of aluminium cylinders with cast in, iron liners, which are "sandwiched" between the cylinder heads and crankcase using long cylinder studs. Previous engines used cast iron cylinders fixed by the base flange. Aluminium, chosen for it's greater ability to disperse heat, is not as rigid as cast iron, and in a Harley engine, distorts when the heads are torqued down. This distortion, which is quite predictable when using the correct gaskets and accurate head bolt torque sequence, absolutely must be mirrored during the cylinder machining processes. The way we do this is with torque plates, which are basically steel plates that the cylinder is sandwiched between whilst machining or measuring. All Harley-Davidson Evolution Big Twin, Evolution Sportster, and Twin Cam cylinders must be machined, and measured, between torque plates. This also includes all after market aluminium cylinders. Even the older, cast iron, Shovelhead cylinders distort around the cylinder base flange when torqued down to the crankcase. For that reason, we also use a lower torque plate when honing these, as well as coated steel base gaskets, which cause less distortion than the soft fibre gaskets commonly used.

Harley-Davidson Twin Cam cylinders, as well as Evolution Big Twin, Evolution Sportster, Buell, and all aftermarket aluminium cylinders, must be sandwiched between torque plates before any measurements or machining can take place. Our torque plates were also designed to fit the fixture that we also designed and built for our honing machine.

We have designed and manufactured our own torque plates. Since there are a number of different cylinder stud bolt patterns, as well as different bore sizes, we needed to manufacture quite a few. These cover all Harley engines from the 45 Flat Head through to big bore Twin Cams. These kept our steel supplier happy and our lathe busy for a while!

In order for the torque plates to do what they were designed for, they must be accurately and sequentially torqued to impart the same distortion to the cylinders that would occur when they are ultimately fitted to the engine. Now we can finally start measuring!

Measuring Harley-Davidson cylinder bores

Forget about these old wives tales of stuffing feeler gauges between the piston and cylinder bore, it doesn't work. Even a conventional internal micrometer seriously struggles here. We need an accurate, to .0001", dial bore gauge, normally found in engineering inspection departments. Unfortunately, these are designed to be used in a clean environment, and the honing operation isn't a clean one. The only bore gauge capable of this accuracy, in these conditions, is made by Sunnen in the USA. (Sunnen, incidently, also pioneered the honing process). A dial bore gauge is what is known as a comparator. That is, it does not give an absolute measurement as it's range is too limited, it compares one size with another. Therefore we also need a way of setting the bore gauge to a known size (the cylinder bore that we wish to measure). Here we use another Sunnen product, a bench micrometer, that also reads directly to .0001". We also need to measure the pistons, and for this we use conventional outside micrometers. These are calibrated to the same setting standards that the bench micrometer uses, so all of our measurements are correct, relative to each other.

So, measure the piston, add the desired clearance, set the bench micrometer to this size, use this to set and "zero" the dial bore gauge, and then measure the cylinders. This in itself would be perfect if every piston that we used here, could be measured with the outside micrometers. However, some pistons can't be measured this way. The Harley-Davidson Evolution Big Twin and Evolution Sportster pistons have a unique compound shape to the piston skirt, which means that the micrometer does not give an accurate measurement of it's dimension. Similarly, some aftermarket pistons, such as some of the big bore stroker pistons from S&S, have part of the skirt machined away on the front of the rear piston, in order to provide piston to piston clearance at the bottom of the stroke. The part that is machined away is where the measurement needs to be taken. In these circumstances we need to rely on the manufacturers' figures regarding either the cylinder bore size or the actual piston dimensions that they supply. For this reason, the setting standards that we use here to calibrate our measuring equipment, are themselves calibrated to an international standard also used by our suppliers.

This is the equipment we use for accurately measuring Harley-Davidson cylinders and pistons. Accurate to a twentieth of the thickness of the cigarette papers we used to set our points ignitions with. The cost of this measuring equipment is about the same as a new Twin Cam engine! The cost of not having it, however, would be some serious reliability issues.

Once the desired cylinder bore size is established, the bench micrometer (that reads directly to one ten thousandth of an inch), is used to set the dial bore gauge. This bore gauge self centres itself in the cylinder bore using a sprung loaded carriage. This carriage also locates it in the setting fixture.

Dial bore gauge is then set to zero, and this then measures any deviation from this known dimension. Each large division is one thousandth of an inch (.001"), and each smaller division is one ten thousandth of an inch (.0001").

Now that we have the dial bore gauge set, it is extremely easy to take very precise measurements within this Twin Cam cylinder. We need to take a number of measurements to determine if there is any taper, bell mouthing, barrelling, or ovality, as well as the bore diameter. The gauge centres itself with a sprung loaded carriage and we merely have to rock it gently back and forth. The lowest reading is the true diameter. Although this equipment is seriously expensive, it makes short and easy work of checking these critical dimensions.